Implement identity graph stitching and configurable profile storage

Issue persistent signed anonymous IDs and accept cookie/token-bound events.
Resolve verified email, phone, OAuth, and namespaced third-party IDs into
transactionally merged master identities with historical event aliases.
Protect shared-device account switching, tenant isolation, and event retries.

Store sparse canonical profiles as PostgreSQL JSONB or local SQLite JSON.
Support an optional versioned Cassandra wide-column projection through a
transactional outbox, retry worker, and immediate canonical read fallback.
Document deployment settings and provide opt-in database integration tests.

Validation: 43 local tests passed; 6 external database tests skipped.
Ruff checks, formatting, and offline uv lock verification passed.

scripts/init_local_env.py

@@ -0,0 +1,21 @@
+"""Create ignored local-development secrets without printing their values."""
+
+import os
+import secrets
+from pathlib import Path
+
+root = Path(__file__).resolve().parents[1]
+text = (root / ".env.example").read_text()
+text = text.replace(
+    "IDENTITY_SECRET=\n", "IDENTITY_SECRET=" + secrets.token_urlsafe(48) + "\n"
+)
+text = text.replace(
+    "SERVICE_TOKEN=\n", "SERVICE_TOKEN=" + secrets.token_urlsafe(48) + "\n"
+)
+try:
+    descriptor = os.open(root / ".env", os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o600)
+except FileExistsError:
+    raise SystemExit(".env already exists; left unchanged") from None
+with os.fdopen(descriptor, "w") as output:
+    output.write(text)
+print("Created local .env with generated secrets. Values are not printed.")